1. Field of the Invention
This invention relates to a method and an apparatus for detecting a reference position of a rotating scale, and more particularly, to a method and an apparatus for detecting a reference position used for a rotary encoder for reading the displacement of a grating formed along the circumferential direction of a rotating scale.
Heretofore, as a measuring instrument which can measure the displacement of an object to be measured in the unit of submicrometers, there has been known an optical encoder in which a laser beam is irradiated on a diffraction grating (scale) linked to an object to be measured, an interference light is formed by superimposing a pair of diffracted lights produced by the diffraction grating with each other, and the displacement of the diffraction grating, that is, the displacement of the object to be measured is measured according to a signal obtained by photoelectrically converting the interference light.
The present applicant has disclosed a method for detecting a reference position (an origin) of a scale in this kind of encoder in Japanese Patent Public Disclosure (Kokai) No. 62-200223 (1987). In the method disclosed by this publication, a mark for detecting a reference position formed on a scale is optically detected, and the reference position of the scale is detected with a very high resolution. Although this method can be applied to a linear scale and a rotary scale, our investigations show that problems sometimes arise, especially when this method is applied to a rotary scale mounted to a small rotating mechanism.
In the method disclosed by the above-described publication, a mark, or marks, consisting of a rectangular reflective film is, for example, provided on a rotating scale, and a rectangular or an elliptic beam spot extended in the direction of the radius of the scale is formed on the scale by irradiating a laser beam on the scale. A reflected light beam produced when the mark crosses the beam spot is then detected by two photosensors disposed in different positions so that detection timings for the reflected light beam are different to each other, levels of output signals from the two photosensors are compared with each other, and the reference position of the scale is detected with excellent accuracy based on the levels of the output signals.
However, if a deviation of the scale or an inclination due to a slanting of the rotation shaft occurs in the rotating scale during its rotation, the detection timings for the reflected light by the two photodetectors shift in the same manner, as shown in FIG. 1, FIG. 2 and FIG. 3, and the reference position is misdetected.
In FIGS. 1 and 2, there is shown a laser diode 1, a collimating lens 2, a half-mirror 3, a cylindrical lens 4, a rotating scale 5, a mark 6 for detecting a reference position, a rotation shaft 7 of the rotating scale 5, and photosensors 8 and 9. Light from the laser diode 1 is made a parallel light by the collimating lens 2, and the parallel light is directed to the half-mirror 3. The parallel light reflected by the half-mirror 3 is converted into a linear light beam by the cylindrical lens 4, and a linear beam spot extended in the direction of the radius of the scale is formed on the scale 5. When the mark 6 passes through the linear spot, the light beam reflected by the mark 6 is directed to the photosensors 8 and 9 via the cylindrical lens 4 and the half-mirror 3.
The mark 6, the linear beam spot and the photosensors 8 and 9 are set so that levels of output signals A and B from the photosensors 8 and 9 coincide with each other only when the center line of the mark 6 and the center line of the linear beam spot coincide with each other, and detection of the reference position of the rotating scale 5 is performed according to a coincidence of output signals A and B from the photosensors 8 and 9. Now, if the rotating scale 5 shifts 0.5 .mu.m in the direction of the X--X' axis which orthogonally crosses the rotation shaft of the scale and is displaced from the position indicated by the full line to the position indicated by the dotted line, as shown in FIG. 1, detection timings of the reflected light beam from the rectangular mark 6 on the rotating scale 5 by the photosensors 8 and 9 shift as shown in FIG. 3. That is, the timing t.sub.1 at which levels of the output signals A and B of the photosensors 8 and 9 coincide with each other shifts to t.sub.1 '. Pulses depicted at an upper portion of FIG. 3 are reference (origin) signals corresponding to the reference position of the scale 5, produced according to a coincidence of levels of the output signals A and B. The reference signal is misdetected by the deviation of the scale 5.
The shift from the timing t.sub.1 to the timing t.sub.1 ' corresponds to the shift of the reference position of the scale 5 by 0.5 .mu.m along the circumferential direction of the scale 5. Hence, when the diameter of the scale 5 is 20 mm, an error of {tan.sup.-1 (0.5/10000).times.3600}.congruent. about 10 angular seconds is produced.
On the other hand, if the rotation shaft 7 of the scale 5 is slanted to tilt the scale 5 as shown in FIG. 2, the incident positions of the light beam reflected by the mark 6 on the photosensors 8 and 9 are shifted, and output timings of the output signals A and B from the photosensors 8 and 9 are shifted. For example, when a lens having a focal length f=5 mm is used as the cylindrical lens 4, the error in detecting the reference position becomes tan.sup.-1 {5.times.tan (1/60.times.2)/10}.times.60=20 angular seconds, for the tilt of the rotation shaft 7 of 20 angular seconds.
In general, when a rotating scale is mounted to a small rotating mechanism using a bearing and the like, it is very difficult to prevent deviation of the shaft of 0.5 .mu.m or less, or prevent slanting of the shaft of 10 angular seconds or less.
Accordingly, in conventional methods, it is difficult to exactly detect a reference position of a rotating scale.